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Calibration of Digital Negatives [1: Monitor]
Hi Garet,
I am not the "David" whom you wrote to, but I will respond anyway. :-)
I used to respond more to digital thread. Sometimes it got too technical, so
people said that my posts were garbage, junk, verbose, irrelavent, etc. so I
have developed a phobia for replying about anything digital, and so I haven't
reply to digital thread for a while; but since we haven't had digital for a
while, I sort of build up my courage again.
But the real reason is that I see that many are still calibrating their
system the hard way (or re-inventing the wheel, so to speak) when we have
Adobe working version after version trying to perfect the calibrating system.
What they have right now is more than enough for our use already, so there is
no reason for re-inventing the wheel.
First to answer your question, yes, at RGB=128 (50%), the negative should
print 50%. It takes some calibration to achieve that so that it *print* right
and *look* right on monitor. We can, of course, look at the monitor and see
that at RGB=128, the area looks light but the print looks dark, so we find an
area in the print that appear like the density that he wants and make a curve
so that at RGB=128, the density of the negative is not 50%. You can calibrate
anyway you want. The disadvantage is now you have to manually handle
everything yourself. You basically disable the nice calibration features that
Photoshop is giving you.
I will attempt to explain the concept of calibration. Some of them you might
know very well already, but since I am targetting a general audience, I
apologize if there are descriptions which is too simple. It will be a little
long. I could skip the details and put down only the steps, but I feel that
such step-by-step approach is already covered in Photoshop's manual but
somehow is still difficult for some to digest, and I also feel that once one
truly understand the concept before the process, the whole thing will become
*very* easy.
The process of calibration consists of 2 important part: how an image looks
on the monitor and how it looks as a print. The process is an iterative one,
so it's hard to describe which comes first, but let's start anyway....
First of all, *define* your black and white point. The industry standard is
to use 10% to 90%. That means RGB value of about 25 to 230. If you want to,
you could also use 5% to 95% (RGB value of about 10 to 240). This actually
depends on your process. You could make print from a digital scale and find
out. For the rest of this message, I will use the first set of values (25 to
230) as my example.
I use "define" to mean that we ***want*** the RGB of 25 to represent darkest
shadow (where you just lose texture, zone 1 if you like) and RGB of 230 to
represent diffuse highlight (zone 9 if you like).
But that is what we ***want.*** What we want might not be what we are getting
yet. Our monitor might not show what we want, so we first need to calibrate
our monitor. First, set your monitor gamma to 1.8 (since the output will be
print, the gamma of 1.8 is about right, you might need to adjust it slightly
later, but for first iteration, remember to set the gamma first before you do
the rest).
First we need to calibrate the "black point." Make a scale from 0 to 255 with
a difference of, say, 5, in each step. Now, turn the brightness control knob
of your monitor, first turn in bright so that you can see good separation
between rgb=20 to rgb=25 (your black point). Gradually turn down the
brightness until you just lose the separation. Your black point is defined.
Next we need to calibrate the "white point." Turn the contrast contrast knob
of your monitor. First turn it to low contrast so that you can clearly see
the separation around RGB=230 (your white point). Now increase your contrast
by turning the knob until you just lose the separation at 230. Your white
point is now defined.
Now your monitor is calibrated to show what you *want* in highlights and
shadows. Midtone might not be right yet (in fact, most probably it is not
right). We will get to that later as we can only set that once we start
scanning and printing.
Wow, I am typing this online and just realized that I have only covered the
monitor part. I will put the calibration of print and the use of Photoshop's
dot gain and gamma in another section or sections. Got to eat lunch now!
Dave Soemarko
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